Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an ac-side connected to a high voltage transmission line or a medium voltage distribution line in an electrical power system and a dc-side connected to a temporary electric power storage means such as capacitors. The STATCOM transforms a dc-voltage on its dc-side into an ac-voltage on its ac-side and can from the power system be seen as a voltage source with variable amplitude and phase angle. The STATCOM can supply reactive power to, or absorb reactive power from, the transmission line or distribution line independently of the voltage of the line.
In one type of multilevel VSC each phase includes a plurality of single phase full-bridge converters connected in series. These single phase full-bridge converters are sometimes referred to as chain-link cells and are in the following denoted cell modules. In FIG. 1 the cell module 6 includes four valves 1-4, each valve including a transistor switch, such as an insulated gate bipolar transistor (IGBT). It is noted that other semiconductor switching devices could be used, for example gate turn-off thyristors (GTO) or integrated gate commutated thyristors (IGCT). A free-wheeling diode, also denoted anti-parallel diode, is connected in parallel with each transistor switch and conducts in the opposite direction of the switch. The valves 1-4 are connected in an H-bridge arrangement with a capacitor unit 5. The cell module further includes a valve controller 11 adapted to control the valves in order to transform a dc-voltage on its dc-side into an ac-voltage on its ac-side.
In comparison with a conventional two-level or three-level VSC, smoother ac current and ac-voltage waveforms are possible to obtain with lower switching frequency and minimal filtering. Each phase of the multilevel VSC includes a number of series-connected cell modules and a line inductor connected in series with the cell modules for current control and filtering purposes. The number of cell modules is proportional to the ac-voltage level of the transmission line or distribution line to which it is connected. Consequently, the VSC can include a large number of cell modules in series. In FIG. 2 is shown one phase of such a multilevel converter connected to the high voltage transmission line or medium voltage distribution line 7 in an electrical power system. The phases of the VSC can be connected in a delta-arrangement as well as in a wye-arrangement. In this example the phase includes four cell modules 6 connected in series with a line inductor 8. Each cell module 6 in FIG. 2 includes a capacitor unit 5 and a plurality of electrical valves. The VSC includes a control unit configured to control the valves according to a switching pattern, for example by using a suitable pulse width modulation (PWM) technique, in order to transform the dc-voltage on its dc-side into an ac-voltage on its ac-side. Each valve is switched on and off a number of times during a fundamental frequency cycle of the ac system. By controlling the timing of the switching within such fundamental frequency cycles, the cell modules provide a desired ac-voltage, being the sum of the ac-voltages of each cell module.
As a large number of cells may be used in series to achieve the ac-voltage level of the transmission line or distribution line, a failure in a single cell module could lead to a necessitated shut-down of the entire VCS if no measures is taken. Consequently, to provide high reliability and availability of the VSC, some type of bypass arrangement is used to be able to continue operation of the VSC. A number of redundant cell modules are provided to replace failed cell modules. If the system is kept operational for the duration of a service interval, the failed modules can be replaced during a scheduled maintenance.
To be able to bypass a faulty cell module, it is necessary to provide zero voltage across the ac terminals of the cell. This can be achieved by using a very fast mechanical switch, a solid-state switch or a combination of both to allow for low power losses.
One example of a converter including a series connection of cell modules and a short circuit device is disclosed in WO-2008/125494 where each cell module of the converter is associated with a short circuit device, e.g. a vacuum switching tube, for short circuiting the cell module. The short circuit device enables safe bridging of a defective cell module.
A problem with mentioned solutions for bypassing failed cell modules is the interruption of the load current i.e., the delay between the failure of the cell module and the bypass performed by the switch. When a cell module fails and goes into an open circuit the load current is interrupted which in combination with a high circuit loop inductance will result in a high voltage across the cell module and extreme energy development which could destroy adjacent equipment.